The bank said that the entire Royal Bank of Scotland Group, of which NatWest is part, would refuse to service RT.

The letter said the decision was final and that it is “not prepared to enter into any discussion in relation to it.”

The RT press office pointed out that NatWest is part of the Royal Bank of Scotland Group, which counts the UK government as a significant shareholder.

“This decision is incomprehensible, and without warning. It is however, not at odds with the countless measures that have been undertaken in the UK and Europe over the last few years to ostracize, shout down, or downright impede the work of RT. RT UK will continue its operations uninterrupted,” RT press office stated.

“We have no idea why it happened, because neither yesterday nor the day before yesterday, nor a month ago, nothing special happened to us, nobody threatened us in any way. Hypothetically, this may have something to do with new British and American sanctions against Russia, which may be announced soon. It may not. Our legal department is dealing with the issue now,” Margarita Simonyan told RBK business news website.

In a statement, the Russian Foreign Ministry expressed its “deep concern” over the bank's move and said that it would ask British authorities “to clarify the situation.”

The bank’s move creates “formidable obstacles for the normal work of the [RT] channel in the UK,” the ministry said, adding that it raises suspicions that it was “an action aimed at eliminating a media outlet, which was unfavorable for the British establishment but popular among the British public.”

“If so, then it is a gross violation of the basic principles of free speech and free press by… ‘the oldest democracy in the world’,” the ministry statement added.

“We will demand explanations from the British authorities for this situation. We will help RT staff to protect their rights. We will request that international organizations like the Council of Europe and the UN, as well as international human rights and media professionals communities state their positions on this issue,” he said.

“I sincerely hope that there’s no political motive for this, because we know that the British government isn’t happy with RT in Britain,” publisher Marcus Papadopoulos told RT.

“RT has a lot of viewers in Britain. Many British people now tune in to RT to receive information on major topics around the world, including in Britain. And many British people believe that the alternative accounts that RT puts forward and covers are more truthful than what they’re hearing from, for example, Sky News.

“If no bank in Britain would allow RT to be a customer, then that could spell the end of RT broadcasting in Britain, which would be a catastrophic event for freedom of speech in Britain,” he added.

“It seems more than a coincidence that this has taken place at a time when the anti-Russian propaganda has been ramped up to unprecedented levels,” writer John Wight told RT. “This reflects the extent to which the West is losing the information war. RT plays a key role in challenging the narrative of the West and Western media when it comes to events in Ukraine, Syria and the Middle East.”

The British government indeed considers RT to be a major security threat, according to a report it gave to the parliament in September. The cabinet’s plan to counter it, as stated to MPs, is to encourage “greater alignment between policy and communications divisions” of NATO structures as well as pumping more money into BBC World Service.